The present invention relates to a stretch-blow-molded container which is obtained by biaxially stretching and blow-molding a thermoplastic polyester and which exhibits heat resistance or heat resistance and pressure resistance to withstand sterilization by heating at the time of hot filling.
Containers obtained by biaxially stretching and blow-molding a thermoplastic polyester such as polyethylene terephthalate (PET) exhibit excellent transparency and surface luster, as well as shock resistance, rigidity and gas-barrier property required for the bottles, and have been used as bottle containers, i.e., as bottles for containing various kinds of liquids.
In producing bottled products, in general, it is necessary to effect the sterilization or pasteurization by heating after a content such as juice, tea or the like has been hot-filled or after a content such as carbonated beverage containing juice has been filled, in order to enhance preservability of the content. However, the bottles made of a polyester have poor heat resistance and undergo a deformation or undergo a contraction causing the volume to change due to heat at the time of hot-filling the content or at the time of sterilization or pasteurization by heating. It is therefore an accepted practice to subject the biaxially stretched blow-molded containers to the step of heat-setting.
Japanese Examined Patent Publication (Kokoku) No. 22862/1994 discloses a heat resistant and pressure resistant container with a semispherical bottom portion, which is provided with a base cup to impart self-standing performance. According to this method of producing containers, a preform having a bottom portion and a mouth-and-neck portion spherulized by heating is biaxially stretched and blow-molded to stretch the whole container at a high stretching ratio except the spherulized portions. Upon effecting the stretch-blow-molding while holding the spherulized bottom portion of the preform with a stretching rod, in particular, the thickness can be decreased up to the peripheral edges of the central spherulized portion in the bottom in a sufficiently highly stretched state. The container having a semispherical bottom portion of which the thickness is decreased in the highly stretched state, exhibits excellent resistance against the heat and pressure, and is capable of sufficiently withstanding the processing of filling a content such as a juice-containing carbonated beverage that must be sterilized and that is likely to produce a pressure, and withstanding the processing of heat-sterilization which flows hot water onto the filled content (according to regulations, at 65xc2x0 C. for not shorter than 10 minutes).
Further, Japanese Unexamined Patent Publication (Kokai) No. 42586/1993 discloses a method of producing a plastic bottle having a self-standing bottom portion relying upon a two-step blow-molding method which includes a primary biaxial-stretch-blow molding and a secondary biaxial-stretch-blow molding. In the Working Examples thereof, there has been described that the thickness of the bottom portion can be decreased to 1.6 mm at the central portion, and the thickness can be decreased to 1.16 to 0.36 mm from the center of the bottom through up to the body portion.
Japanese Patent No. 2917851 proposed by the present applicant teaches a method of producing heat resistant and pressure resistant self-standing plastic container which is obtained by blow-molding a preform-molded article heated at a stretching temperature, and of which the bottom portion includes a plurality of foot portions and valley portions, the valley portions substantially forming part of a semispherical surface, wherein a method of producing biaxially stretched plastic bottles having excellent heat resistance and pressure resistance, comprises:
a step of obtaining a secondary molded article by biaxially stretching and blow-molding the preform-molded article, the portion of said secondary blow-molded article that becomes the bottom portion of the final container having a surface area larger than the surface area of said semispherical surface and, further, having a roughly dome-shaped bottom portion which is relatively highly stretched to possess a thickness of not larger than 1 mm except the central portion of the bottom;
a step of obtaining a tertiary molded article by causing part of the bottom portion of said secondary molded article and part of the body portion continuous to the bottom portion to be opposed to an infrared-ray radiating member, so that the opposing portions are contracted by heating, the portion that becomes the bottom portion of the final container having a such size that can be held in the semispherical surface and is relatively close to the semispherical surface; and
a step of obtaining a final product by subjecting the tertiary molded article in a heated state to the secondary blow-molding in a metal mold.
The container having the semispherical bottom portion of a decreased thickness due to stretching and, particularly, having a spherulized central portion of the bottom, exhibits excellent resistance against the heat and pressure, and is capable of sufficiently withstanding the processing for filling the content such as a juice-containing carbonated beverage that may produce a pressure and the heat-sterilization processing by pouring hot water onto the filled content (according to regulations, at 65xc2x0 C. for not shorter than 10 minutes) requiring, however, a cumbersome operation of forming a base cup separately from the container and securing it to the container with an adhesive. Further, the container of which the central portion of the bottom is crystallized is accompanied by such problems as deteriorated appearance and giving an incorrect impression of sedimentation of the content to the consumers.
The plastic bottle having a plurality of foot portions and a plurality of valley portions that are alternatingly arranged, and having a thickness which is very decreased due to stretching except the central portion of the bottom, exhibits advantages of excellent resistance against the heat and pressure and excellent self-standing performance even after heat-sterilization. However, this plastic bottle is still accompanied by such problems as insufficient degree of stretching at the centeral portion of the bottom, insufficient degree of crystallization, imposing limitation on its heat resistance and pressure resistance, and leaving much room for improvement.
In studying a container obtained by stretch-blow-molding a polyester, the present inventors have discovered that a pseudo crystalline structure which is independent from the crystals inherent in the polyester can be introduced into at least the central portion of the bottom, and that introduction of the pseudo crystalline structure helps improve the heat resistance at temperatures lower than the melting point of the pseudo crystals and, hence, strikingly improve the heat resistance or the resistance against heat and pressure, as well as the self-standing performance of the container after the sterilization particularly when the container has a bottom portion constituted by a plurality of foot portions and a plurality of valley portions.
According to the present invention, there is provided a stretch-blow-molded container obtained by stretch-blow-molding a preform formed of a thermoplastic polyester as a chief component, wherein the body portion, bottom portion and shoulder portion are stretch-oriented, and at least the central portion in the bottom has an endothermic peak (B) due to the extinction of pseudo crystals in a temperature region higher than a glass transition point of the polyester but lower than a temperature at which the crystals of the polyester start melting as measured by using a differential scanning calorimeter (DSC).
In the stretch-blow-molded container of the present invention, it is desired that:
1. An endothermic amount at the endothermic peak (B) due to the extinction of the pseudo crystals is not smaller than 8% and, particularly, from 10 to 20% of the melting endothermic amount of the crystals;
2. The shoulder portion, too, has an endothermic peak (S) due to the extinction of the pseudo crystals in a temperature region higher than the glass transition point of the polyester but lower than the temperature at which the crystals of the polyester start melting as measured by using the differential scanning calorimeter (DSC);
3. An endothermic amount at the endothermic peak (S) due to the extinction of the pseudo crystals is not smaller than 5% and, particularly, from 7 to 15% of the melting endothermic amount of the crystals;
4. An endothermic amount at the endothermic peak (B) due to the extinction of the pseudo crystals is the greatest in the central portion of the bottom, next greatest in the shoulder portion and is the smallest in the body portion in a temperature region higher than the glass transition point of the polyester but lower than the temperature at which the crystals of the polyester start melting;
5. The container is obtained through a primary blow-molding of the preform, through the heat-treatment of the bottom portion, body portion and shoulder portion of the primary blow-molded article, and through the secondary blow-molding of the heat-treated article; and
6. The bottom portion is constituted by foot portions and valley portions alternatingly arranged in the circumferential direction, the valley portions having a self-standing structure located on an imaginary curved plane protruding downward in the axial direction of the container.